This invention pertains generally to the field of fiber optics. In particular, it pertains to glass compositions which improve the performance of optical fiber lasers, amplifiers, and superluminescent sources used in telecommunications and sensor systems.
It has been a long term goal for telecommunications systems to incorporate active functions into optical fiber rather than have the fiber act only as a passive waveguiding medium. In particular, the function of direct optical amplification within the core of a fiber has many potential advantages if a practical means could be devised of transferring optical pump power to signal power. Other important applications of active fibers are as lasers and superluminescent sources. The latter device is a high power source of incoherent, broadband light; a description of it can be found in U.S. Pat. No. 4,637,025 by Snitzer et al. Rare-earth doped fiber lasers are of interest in telecommunications because they are readily adapted to produce output with a very narrow linewidth and a high percentage of their output can be coupled into the singlemode transmission fiber. Fiber lasers and superluminescent sources are potentially important in sensor technology for the latter reason also, and because they can be made to operate at a variety of important wavelengths. The same physical process, i.e. stimulated emission of photons, governs the behaviors of lasers, optical amplifiers, and superluminescent sources. The discussion of this invention focuses on the optical amplifier since the efficient production of the high single-pass gain required for such a device makes it the most sensitive to the factors limiting pump efficiency. Nevertheless, all the considerations for optical amplifiers other than signal-to-noise ratio (SNR) apply as well to lasers and superluminescent sources, and the corresponding benefits should be obvious to one skilled in the art.
Telecommunication systems make use of the low attenuation optical windows that are located at 1.3 and 1.55 microns in silica fiber. The use of the rare earth ions Er.sup.3+ and Nd.sup.3+, which have luminescent transitions falling within these windows, as fiber dopants is known in the art, as disclosed in "Excited State Absorption of Rare Earths in Fiber Amplifiers", L. J. Andrews et al, Conference Proceedings, International School on Excited States of Transmission Elements, Wroclaw, Poland, June 20-25, 1988, which is incorporated herein by reference as background material.
Direct amplification of 1.5 .mu.m optical signals within an optical fiber has been successfully demonstrated in a number of laboratories using fiber which has been doped in the core with the rare earth ion Er.sup.3+. The mechanism for light amplification in doped fibers is exactly the same as for laser action; it is a result of stimulated emission from the excited rare earth ion induced by the optical signal as it propagates through the fiber core. The energy stored in an inverted population of excited Er.sup.3+ ions is transferred to the signal, causing the signal to experience an increase in optical power (gain). The population inversion required for this effect is brought about by an optical pump, a second light source coupled to the fiber core which is of greater intensity than the signal and which is resonant with one of the Er.sup.3+ absorption bands. The overall efficiency with which the optical pump power is transferred to signal power depends upon the coupling between the pump light and the Er.sup.3+ ions, and the coupling between the Er.sup.3+ ions and the signal light. This efficiency has been found by a number of researchers to be highly dependent on which Er.sup.3+ absorption band is pumped. This phenomenon is the principal limitation of the amplifier and results from inefficient coupling between the pump light and the Er.sup.3+ ions. We have identified a way to increase the efficiency of this coupling for the important pump wavelengths near 800 nm and significantly improve the performance of the amplifier when pumped by AlGaAs diode lasers.
The Er.sup.3+ optical absorption spectrum is comprised of a number of transitions, including eight which lie in the visible to near-infrared spectral region. Optical pumping any of them will cause the Er.sup.3+ ion to luminescence at 1.5 microns, a wavelength which happens to coincide with the "third" telecommunications window in silica fiber. This fact is the origin of telecommunication interest in Er.sup.3+ doped fiber. To date, optical amplification has been demonstrated at 1.5 microns through laser pumping most of these transitions, and Table 1 contains a summary of recently reported results.
TABLE 1 ______________________________________ PUMP WAVELENGTH EFFICIENCY Pump Pump Wavelength Power Gain Efficiency (nm) (nm) (dB) (dB/mW) ______________________________________ 514.5 225 33 0.15 532 25 34 1.36 665 100 26 0.26 807 20 8 0.40 980 11 24 2.18 1490 36 14.4 0.40 ______________________________________
Of particular interest is the efficiency value quoted in Table 1 for pumping at 807 nm, the nominal operating wavelength of AlGaAs/GaAs laser diodes. These laser diodes are by a wide margin the best developed semiconductor pump sources and any practical Er.sup.3+ fiber amplifier will require such a high power, reliable, and inexpensive pump. However, the efficiency of 0.4 dB/mw reported for 800 nm pumping is substantially lower than the best efficiency shown in Table 1 even though this value was obtained using a fiber specially designed to enhance performance. In addition to gain, signal-to-noise ratio (SNR) is a major consideration in optical amplifiers, and here, as well, pumping at 800 nm is expected to yield poorer performance than pumping at other Er.sup.3+ absorption bands. The performance of 800 nm pumped amplifiers must be improved in order to develop practical devices. This disclosure addresses this problem and describes a method for increasing the 800 nm pumping efficiency of Er.sup.3+ fiber amplifiers, lasers, and superluminescent sources.
The Er.sup.3+ 800 nm pump band has low efficiency due to a combination of strong excited state absorption (ESA) and weak ground state absorption (GSA). This has been confirmed by direct spectroscopic observation by us and earlier by others, as well as through models of fiber amplifiers. The effect is straight-forward to understand. The GSA spectrum of a transparent material containing Er.sup.3+ ions consists of a series of bands arising from transitions from the ground state of the ion to the various excited states. In the case of the Er.sup.3+ pump band at 800 nm, the transition is from the .sup.4 I.sub.15/2 ground state to the .sup.4 I.sub.9/2 exited state. Under conditions of intense optical pumping, the lowest excited state of Er.sup.3+ (.sup.4 I.sub.13/2) becomes appreciably populated, even to the extent of having a much higher population than the ground state. This is the population inversion required to achieve gain. Under conditions of high inversion, the absorption spectrum changes to that of the ESA spectrum and now consists of transitions from the lowest excited state to the higher excited states. It turns out coincidentally that the GSA spectrum and the ESA spectrum both have bands at 800 nm. For the silica glass fiber materials that have been examined in the literature to date, the spectrally integrated ESA band intensity exceeds the integrated GSA band intensity at 800 nm by a factor of two. This means that as the Er.sup.3+ excited state population increases under optical pumping at 800 nm, pump photons will be preferentially absorbed by the excited state rather than the ground state. In silica glass, the higher excited states quickly decay to the lowest excited state through the liberation of heat. This nonsaturable parasitic process can lead to serious pump inefficiency because pump photons merely recycle excited states causing heat production, and are lost for doing the useful work of transferring Er.sup.3+ population from the ground to the first excited state.